This invention relates to a process for continuously producing a foamed sheet. More specifically, the invention relates to a process which can be advantageously used for producing a foamed sheet by heat-foaming a thermoplastic resin sheet having the property of foaming upon heating.
A technique for producing a foamed sheet has been widely known heretofore which comprises applying ionizing radiation, organic peroxides, etc. to a sheet made of a thermoplastic resin such as polyethylene containing an organic blowing agent, and then heating the resulting cross-linked sheet.
In the continuous production of a foamed sheet, the general practice has been a "horizontal-type" foaming method in which an unfoamed sheet is heated while it is being conveyed horizontally on a belt conveyor. This method, however, has the defect that during the three-dimensional expansion of the unfoamed sheet, the foamed sheet sticks to the belt conveyor, and its free expansion is impeded to cause non-uniformity in thickness and an increase in wrinkles. As another defect, the surface condition of the foamed sheet differs between that surface which makes contact with the belt conveyor and the uncontacted surface, and the commercial value of the sheet is thus reduced. In an attempt to remedy these defects, it has been suggested to cause the sheet to be afloat on a hot liquid bath such as ethylene glycol, or to support the sheet by applying hot air thereto from under the sheet. These methods, however, encounter considerable difficulties, and can scarcely remove the aforesaid defects.
A "vertical-type" foaming apparatus, such as disclosed in U.S. Pat. No. 3,711,584, is also known. This apparatus includes a preheating chamber and a foaming chamber connected to each other vertically with a partitioning apertured wall therebetween, in which a sheet fed from the top of the preheating chamber is foamed while it is caused to descend perpendicularly through the preheating chamber and foaming chamber. With such an apparatus, the sheet does not make contact with a belt conveyor or the like, and can be uniformly heated from both surfaces. Thus, it is convenient for obtaining a uniformly foamed sheet without a difference in condition between the two surfaces.
In such a vertical foaming apparatus, the sheet undergoes preheating while it is suspended from the feed opening of the preheating chamber to the sheet exit of the foaming chamber. The sheet softened by preheating, therefore, expands in the lengthwise direction by its own weight. Foamed sheets obtained by foaming in the extended state have distorted cells, and therefore, shrink during subsequent processing such as heat-welding. In other words, the conventional vertical-type foaming apparatus cannot afford foamed sheets having good dimensional stability. Furthermore, with the conventional vertical foaming apparatus, the sheet undergoes vibration and develops curls because the apparatus is without a supporting device. The vibration and curling are likely to cause the sheet to contact the apparatus. In order to avoid contact between the sheet and the apparatus, it is necessary to enlarge the sheet feed opening of the preheating chamber, the apertures in the partitioning wall between the preheating chamber and the foaming chamber, and the sheet exit of the foaming chamber, and to increase the distance between the sheet and the heating device. As a result, however, the heat of the foaming chamber moves into the preheating chamber, and the heat of the preheating chamber escapes from the sheet feed opening. The sheet efficiency is thus reduced, and it is difficult to control the level of heat in the preheating chamber. Furthermore, since curling of the sheet increases the temperature distribution of the sheet, foaming does not begin uniformly. Thus, expansion is not effected uniformly in the three-dimensional directions, and the individual foamed portions melt-adhere to one another to increase the curling of the end portions of the foamed sheet. This defect becomes greater at higher heating temperatures. Because the inside of the apparatus must be heated to a greater extent in order to increase the output in an apparatus of the same scale by increasing the sheet feeding speed, the above defect means that with the conventional vertical-type foaming apparatus, it is almost impossible to increase the output. In the production of foamed sheets having a large width, on the other hand, the curling of the sheet increases, and therefore, the sheet feed opening must be enlarged. This results in reduced heat efficiency and causes difficulty of heat control. Hence, in order to increase the output in the conventional vertical-type foaming apparatus, it is necessary to increase the lengths of the preheating chamber and the foaming chamber, and the apparatus as a whole naturally becomes large-sized. In a large-sized apparatus, the distance over which the sheet is suspended increases, and the various defects described become more outstanding.